In the field of conductive materials including metal sheets such as steel sheets, attentions have been recently drawn to high functional (high-value-added) materials that have been given various functions on the surface. A particularly attractive method of imparting functions to the surface of conductive materials is to form a fine structure on the order of micrometers or less (nano level) on the material surface so as to give any of various functions on the surface of materials.
With regard to the formation of nano-level fine structures on material surfaces, a method has been studied in which a nanometer-sized fine structure is formed on the surface of a substrate or the like by a photolithographic method or a chemical vapor deposition (CVD) method (for example, see Non Patent Literature 1). However, the formation of a nano-level fine structure by photolithography or chemical vapor deposition entails advanced techniques and expensive devices. Thus, these methods do not allow low-cost and efficient formation of nano-level fine structures.
On the other hand, methods involving submerged electric discharge have been recently proposed for the purpose of forming nanometer-sized fine particles at low cost and efficiently, cleaning a conductive surface, or coating (metalizing) a conductive surface after the surface is cleaned (see, for example, Patent Literatures 1 to 3).
In detail, Patent Literature 1 proposes a method in which a high voltage is applied between a cathode and an anode placed in an electrolytic solution so as to generate plasma by glow discharge in the vicinity of the cathode (hereinafter, referred to as “submerged plasma”), thereby locally fusing the cathode material to form conductive fine particles (nanoparticles) of the cathode material in the solution. In order to form finer particles by submerged plasma discharge, Patent Literature 1 adopts the application of such a voltage that exhibits a complete-state plasma in which the plasma emits light from the entire surface of electrode (for example, a voltage as high as 140 to 300 V, although variable in accordance with, for example, the type of cathode material used).
Patent Literatures 2 and 3 propose methods which include providing a workpiece as a cathode and an anode that has one or more apertures, causing an electrolyte to flow through the apertures in the anode and impinge on the surface of the workpiece, and applying a predetermined voltage between the workpiece (cathode) and the anode so as to clean the surface of the workpiece or further to form a coating on the surface of the workpiece after cleaning. According to Patent Literatures 2 and 3, the surface roughness of the cleaned workpiece is so increased as to achieve high adhesion with respect to a coating film.
However, the techniques disclosed in Patent Literatures 1 to 3 are dedicated to the production of nanoparticles from a cathode material, or to cleaning or coating of the surface of a workpiece. That is, they do not have any technical idea of forming a nano-level fine structure on the surface of a cathode material. Moreover, the fact that electric discharge is performed under conditions which increase the surface roughness of a cathode material (a workpiece) eliminates the possibility for the methods to be used for the formation of a nano-level fine structure on the surface of the cathode material itself.